What the tracheids are to xylem sieve elements are to phloem. Sieve elements may be segregated into relatively less specialized sieve cells (found in pteridophytes) and more specialized sieve tubes. Seive tubes are living cells and have a thin cellulose wall. In angiosperms they are enucleate, while they are nucleate in gymnosperms.
The protoplast of a sieve tube has a big central vacuole which is filled with viscous and albuminous proteins. Leucoplasts are also reported in the vacuole. The cross walls between the seieve tubes are porus and are called sieve plates as they resemble a sieve. Sieve plates are usually of two types viz., simple and compound. In simple sieve plates, the pores are large and the plate is horizontal or transverse in position.
The compound sieve plate is somewhat oblique. The sieve pores are large and the plate is horizontal or transverse in position. The compound sieve plate is somewhat oblique. The sieve pores are very often plugged with a shining substance called callus pad which is usually deposited at the end of the spring season. Chemically callus pads are made up of a carbohydrate called callose. At the approach of the spring, the callus dissolves. Through the sieve pores, thin cytoplasmic strands come out and enter into other cells.